Transport Properties And Vortex Pinning In Cu_xBi₂Se₃ And K_xFe_2-ySe₂ Single Crystals

Topological insulators and iron-based high-temperature superconductors are currently two important and active areas of research in condensed matter physics. Topological insulators are a new class of quantum materials, characterized by their fully insulating gap in the bulk but gapless edge or surface metallic states protected by time-reversal symmetry. Comparing to topological insulator, their “sister” materials-topological superconductors, characterized by a fully gapped in the bulk with odd-parity pairing state, but edge/surface metallic state are still preserved, are also received considerable attentions in both theoretical and experimental works. Theories predict that the Majorana fermion bound states can be generated in the vortex cores in topological superconductors. Majorana fermions are important because they are expected to be used to build a topological quantum computer. As a topological superconductor candidate, although the Cu-doped three-dimensional topological insulator Bi2Se3, namely CuxBi2Se3, have received a lot of theoretical and experimental research, so far,the experimental studies of vortex pinning properties are still very limited. Therefore, it will be vital to carry out experimental study of pinning mechanism in CuxBi2Se3 so as to facilitate to fully understand its unconventional superconductivity and explore Majorana fermions in vortex cores.The research boom of iron-based high temperature superconductors was started by the Kamihara et al’s seminal discovery of superconductivity at 26 K in F-doped LaFeAsO in 2008. At the end of 2010, Guo et al from Institute of Physics, Beijing, discovered a new toxic arsenic-free iron selenide superconductor, KxFe2-ySe2(~30 K). Since then it was attracted broad attention because of its intriguing physical properties distinguished from other iron-based superconductors. It was demonstrated by experimental studies that KxFe2-ySe2 superconductor has stoichiometry/structure phase separation, in which the superconducting phase and insulating phase distribute spatially independent. We studied the effects of Mn doping on the phase separation and vortex pinning mechanism, which can provide experimental references for the synthesis of bulk superconducting samples and applications of related iron selenide superconductors in high magnetic field in the future. The main contents and results are as follows:(1) High quality single crystals of CuxBi2Se3(x=0, 0.20) were grown by modified Bridgman method. By measuring magnetic hysteresis loops and magnetoresistance, for the first time, we observed the anisotropic peak effect, an anomalously large increase/increase/decrease of the magnetization/critical current(density)/resistivity nearby the superconducting-normal phase boundary, in topological superconductor candidate Cu0.10Bi2Se3(~3.65 K). The Cu0.10Bi2Se3 belongs to weak pinning superconductors characterized by a small ratio of critical/depairing current density (0)/0~10-5. Towards increase of magnetic field below “peak effect” temperature for ∥ c, the magnetic vortex phase diagram includes vortex ordered phase(Bragg glass), coexistence of ordered-disordered phase(plastic glass), disordered phase(armorphous glass) and surface superconductivity. For ∥ c, a small Ginzburg number ?< 10-5 and a large quantum resistance u=8.56×10-2 for Cu0.10Bi2Se3 show weak thermal fluctuation but the stronger effect of quantum fluctuation correlated to the vortex dynamics, hence probably resulting in the appearance of “quantum creep” phenonmenon at low temperature.(2) Utilizing magnetotranport measurements, we investigated the temperature/magnetic field/driving current dependence of anisotropic in-plane magnetoresistance(xx) and critical current(c) for Cu0.10Bi2Se3(=3.54 K) single crystal. Its superconductivity is demonstrated to be in clean limit and absence of Pauli spin-limiting effect, thus the spin-triplet vortex state may exist in Cu0.10Bi2Se3. Towards increase of magnetic field below “peak effect” temperature, the analasis of anisotropic vortex dynamics show the magnetic vortex phase diagram mainly includes moving Bragg glass, pinned vortex glass and depinned vortex glass state.(3) In low field region, the () curves of Cu0.10Bi2Se3 single crystal follows a power-law dependence ∝ -, a signature of the weak collective pinning behavior, which implies the existence of a fairly dense distribution of weak pinning centers(pins) in the sample. Scanning tunneling microscopy experiments reveal the weak pins mainly from point defects, probably originating from the intercalated Cu atoms. Furthermore, it is demonstrated that the vortex pinning originates from the carrier mean free length fluctuation(δ pinning). According to the collective pinning theory, we performed a computed analysis of the anomalous vortex pinning behavior, namely, peak effect, in which the longitudinal(transverse) correlation length () as well as correlated volume = 2 were calculated. In the case of three-dimension collective pinning, the peak effect in Cu0.10Bi2Se3 could be described as a result of the softening of elastic moduli of vortex lattices due to the reduction of nonlocal dispersive tilt modulus 44 and the local shear modulus 66 near c2, thus resulting in smaller correlation length of vortex lattices to increase the pinning force rapidly.(4) High quality single crystals of Mn doped K0.8Fe2-zMnzSe2(z=0, 0.01, 0.02, 0.03) have been grown by a melt method. Analysis of crystal structure, electromagnetic transport and micro-surface morphology data clearly shows that Mn doping can enhanced phase separation and improve superconducting performance, namely, increasing the superconducting shielding fraction and critical temperature. Our results indicate that the Mn-dopant may highly favor to fill into the iron vacancies in the insulating phase to induce a local lattice strain or distortion, which can further modify the microstructure of superconducting phase. Furthermore, we performed a comparative analysis of the anisotropic superconducting critical field, anisotropy and coherence length among the pure and Mn doped samples.(5) Based on magnetic hysteresis loops data, we present a comparative study of the critical current density and vortex pinning among pure and Mn doped KxFe2-ySe2 single crystals. It is found the values can be greatly improved by Mn doping and post-quenching treatment when comparing to pristine pure sample. In contrast to pure samples, an second magnetization peak(SMP) effect is observed in both 1% and 2% Mn doped samples at =3 K for ∥ but not for ∥ . Referring to Dew-Hughes and Kramer’s model, we performed scaling analyses of the vortex pinning force density vs magnetic field in 1% Mn doped and quenched pristine crystals. The results show the normal point defects are the dominant pinning sources, which probably originate from the variations of intercalated K atoms. We propose the large nonsuperconducting K-Mn-Se inclusions may play key role of the normal surface pinning centers and give rise to the SMP effect for ∥ in Mn doped samples.